1. Technical Field
The present invention relates to a channel estimation method used for digital communications. The present invention relates particularly to a configuration of pilot channels or to a channel estimation unit in a wireless system that combines MIMO (Multi Input Multi Output) and OFDM (Orthogonal Frequency Division Multiplexing) with each other.
2. Background Art
There have been active researches and studies of a high-speed/large-capacity mobile communication system over the recent years, and the attention has been paid to a system that combines the MIMO and the OFDM with each other as a basic system thereof.
FIG. 16 is a diagram showing a basic configuration of an OFDM transmitter in an OFDM transmission system. Transmission data are mapped as frequency domain data of respective subcarriers by serial/parallel conversion. The frequency domain data undergo inverse Fourier transform into time domain waveform data. Transmitted next is an OFDM symbol organized by attaching (inserting), as a guard interval (GI), part of data of the trailing portion of this time domain to the head (of the symbol).
Herein, an in-depth description of the OFDM transmitter in FIG. 16 is given. A serial/parallel converting unit 101 converts inputted channel data into a frequency domain data of a subcarrier. The serial/parallel converting unit 101 outputs the frequency domain data to an inverse fast Fourier transform operation unit 102. The inverse fast Fourier transform (IFFT) operation unit 102 inverse-Fourier-transforms the inputted frequency domain data into time domain data. The inverse fast Fourier transform operation unit 102 outputs the time domain data to a guard interval inserting unit 103. The guard interval inserting unit 103 attaches (inserts), as the guard interval, part of data of the trailing portion of the time domain to the head (of the symbol). The data attached with the guard interval is defined as an OFDM symbol. The insertion of the guard interval (guard period) can reduce influence of interference, caused by multi-paths, between the OFDM symbols. The guard interval inserting unit 103 outputs the OFDM symbol to a digital/analog converting unit 104. The digital/analog converting unit 104 converts the OFDM symbol inputted from the guard interval inserting unit 103 into analog signals, and outputs the analog signals to an up-converting unit 105. The up-converting unit 105 up-converts the inputted analog signals into high-frequency signals. An amplifier 106 amplifies the high-frequency signals and transmits the amplified signals from the respective antennas 107.
FIG. 17 is a diagram showing a basic configuration of the OFDM receiver in the OFDM transmission system. The received time domain signals undergo extracting the individual OFDM symbols and removing the guard intervals (GIs) at the symbol timings obtained from the correlations of the guard intervals. The GI-removed signals are subjected to fast Fourier transform (FFT) and are thereby separated into the signals according to the subcarrier. The channel estimation unit obtains a channel estimation value for every subcarrier, and channel correction per subcarrier is conducted based on the channel estimation value, thereby obtaining a symbol value per subcarrier.
Herein, the OFDM received in FIG. 17 will be explained in detail. A down-converting unit 202 down-converts the high-frequency signals transmitted from the transmitter and received by the receiving antennas 201. An AGC (Auto Gain Control) amplifier 203 amplifies the down-converted reception signals. An analog/digital converting unit 204 converts the amplified reception signals into digital signals. A symbol timing detection unit 205 detects the timing of the OFDM symbol from the guard interval (GI). A guard interval removing unit 206 removes the guard intervals inserted by the transmitter from the digital signals.
A fast Fourier transform (FFT) operation unit 207 transforms the digital signals in the time domain into a plurality of subcarrier signals in the frequency domain. A channel estimation unit 208 makes channel estimation per subcarrier by use of the pilot signals that are time-multiplexed by the transmitter. A channel estimation value obtained by the channel estimation unit 208 is multiplied by an output of the fast Fourier transform operation unit 207, thereby correcting a channel fluctuation. A parallel/serial converting unit 209 converts the corrected parallel data into the serial data, thereby obtaining the serial channel data.
The channel estimation value is obtained from the pilot symbol of the known signals. FIG. 18 is a diagram showing an example of a frame structure. In this case, the data of four pieces of OFDM symbols are transmitted subsequent to two pieces of OFDM pilot symbols.
The OFDM is capable of greatly suppressing the interference, caused by the multi-paths etc, between the symbols by inserting the GIs. Further, a length of the OFDM symbol is relatively long, resulting in a less decline of the data transmission efficiency due to the insertion of the GIs.
FIG. 19 is a diagram showing the basic architecture of the MIMO. The MIMO provides a plurality of antennas on the transmitting side and the receiving side, respectively (the four antennas are provided on both of the transmitting and receiving sides in FIG. 19). When different pieces of data s0, s1, . . . , sN are transmitted from the respective antennas on the transmitting side, the data are received in the form of these pieces of data being mixed together on the receiving side. Let v0, v1, . . . , vN be the reception signals at that time, a relationship can be expressed as by the Formula (1).
[Mathematical Expression 1]
                              (                                                                      v                  0                                                                                                      v                  1                                                                                    ⋮                                                                                      v                                      N                    -                    1                                                                                )                =                                            (                                                                                          h                      00                                                                                                  h                      01                                                                            ⋯                                                                              h                                              0                        ⁢                                                  (                                                      N                            -                            1                                                    )                                                                                                                                                                                h                      10                                                                                                  h                      11                                                                            ⋯                                                                              h                                              1                        ⁢                                                  (                                                      N                            -                            1                                                    )                                                                                                                                                          ⋮                                                        ⋮                                                        ⋱                                                        ⋮                                                                                                              h                                                                        (                                                      N                            -                            1                                                    )                                                ⁢                        0                                                                                                                        h                                                                        (                                                      N                            -                            1                                                    )                                                ⁢                        1                                                                                                  ⋯                                                                              h                                                                        (                                                      N                            -                            1                                                    )                                                ⁢                                                  (                                                      N                            -                            1                                                    )                                                                                                                                )                        ⁢                          (                                                                                          s                      0                                                                                                                                  s                      1                                                                                                            ⋮                                                                                                              s                                              N                        -                        1                                                                                                        )                                +                      (                                                                                n                    0                                                                                                                    n                    1                                                                                                ⋮                                                                                                  n                                          N                      -                      1                                                                                            )                                              (        1        )            
Here, hij represents a propagation channel from a j-th transmitting antenna to an i-th receiving antenna, and ni represents a noise entering the i-th receiving antenna. To give a vector notation, this is expressed by the Formula (2).[Mathematical Expression 2]v=Hs+n  (2)
Here, when multiplying the reception signal v by an inverse matrix of a channel matrix, the transmission signal can be restored.[Mathematical Expression 3]H−1v=s+H−1n  (3)
Thus, the MIMO can transmit and receive the plurality of symbols and therefore has a possibility of drastically improving the communication capacity (traffic size). Herein, respective elements of a matrix H needed for demodulation are estimated from reception signals of known patterns (pilot symbols) that are transmitted sequentially from the individual antennas. The signals of the respective antennas can not be separated at a point of estimating the matrix H, and hence the pilot symbols need transmitting by time division.
The MIMO has poor compatibility with the interference between the codes which is caused by the multi-paths etc, and is therefore utilized in combination with the OFDM system capable of avoiding this problem in many cases.    [Patent document 1] Japanese Patent Application Laid-Open Publication No. 2005-110130    [Patent document 2] Japanese Patent Application Laid-Open Publication No. 2004-253925    [Patent document 3] Patent Publication for WO 2005/011167